The present invention relates to a front passenger seat airbag apparatus that inflates and deploys an airbag in front of a front passenger seat to protect an occupant seated in the seat from an impact caused by a vehicle collision.
As one means for protecting an occupant seated in a front passenger seat when an impact is applied to the vehicle from the front, a front passenger seat airbag apparatus 80 shown in FIGS. 16, 17(A), and 18(A) is known (for example, refer to Japanese Laid-Open Patent Publication No. 2006-88830 and Japanese Laid-Open Patent Publication No. 2008-110737). FIG. 16 illustrates components of the front passenger seat airbag apparatus 80. FIGS. 17(A) and 18(A) are cross-sectional views of an airbag cover 85 of the front passenger seat airbag apparatus 80, showing parts that are engaged with a case 83. These drawings do not show parts other than the engaging parts. The front passenger seat airbag apparatus 80 includes an airbag 81, an inflator 82 for supplying inflation gas to the airbag 81, the case 83 accommodating the airbag 81 and the inflator 82, a retainer 84 for attaching the airbag 81 and the inflator 82 to the case 83, and the airbag cover 85 covering the airbag 81.
The case 83 is formed by processing a single metal plate. The case 83 includes a bottom portion 86 located underneath the airbag 81 and a tubular inner wall portion 87 having a rectangular tube-like structure surrounding the airbag 81. The tubular inner wall portion 87 includes four inner walls 88, 91 extending upward from the bottom portion 86 of the case 83. A pair of the inner walls 88 are arranged at front and rear sides of the tubular inner wall portion 87, so as to face each other. Each inner wall 88 has a plurality of claw portions 89 extending outward. The claw portions 89 are aligned on the surface of the inner wall 88 along a horizontal direction.
The airbag cover 85 is formed by molding resin, and includes a door portion 92 located above the airbag 81 and a tubular outer wall portion 93 located below the door portion 92. The tubular outer wall portion 93 includes a pair of front and rear outer walls 95 facing each other, and another pair of outer walls 96 facing each other in a lateral direction. There are two types of tubular outer wall portion 93: one type has a slit 97 at each corner, or each boundary between adjacent pair of the outer walls 95, 96 (FIGS. 16 and 17(A)); and the other has no such slits (FIG. 18(A)). In either type, the airbag cover 85 surrounds the tubular inner wall portion 87 with the tubular outer wall portion 93 while covering the upper opening of the tubular inner wall portion 87 with the door portion 92. Then, the claw portions 89 of the inner wall 88 are inserted in and engaged with engaging holes 94 formed in the outer wall 96, so that the airbag cover 85 is engaged with the case 83.
When impact is applied from the front to a vehicle equipped with the above described front passenger seat airbag apparatus 80, the inflator 82 supplies inflation gas to the airbag 81 to start inflating and deploying the airbag 81. When the inflation of the airbag 81 opens the door portion 92, the airbag 81 passes through the opening of the door portion 92 and is inflated and deployed between the instrument panel and the occupant in the front passenger seat, thereby protecting the occupant from the impact.
When being inflated and deployed, the airbag 81 applies a high pressure to each part of the case 83 as shown in FIGS. 17(B) and 18(B). The pressure deforms each of the inner walls 88, 91 outward, causing it to bulge outward in an arched shape. The degree of bulging of each inner wall 88, 91 is the smallest at the boundaries B with the adjacent inner walls 91, 88, and increases as the distance from the boundaries B increases. This is because the rigidity of each inner wall 88, 91 is the greatest at the boundaries B and is reduced as the distance from the boundaries B increases.
The high pressure accompanying the inflation and deployment of the airbag 81 is also applied to the tubular outer wall portion 93 of the airbag cover 85. At this time, the tubular outer wall portion 93 is deformed differently when a slit 97 is formed between each adjacent pair of the outer walls 95, 96 (FIG. 17(B)) from when no such slit 97 is formed (FIG. 18(B)). The deformation of the outer walls 95 in the tubular outer wall portion 93 and the deformation of the inner walls 88 in the tubular inner wall portion 87 cause the following problems.
<When the Slits 97 are Formed in the Tubular Outer Wall Portion 93 (FIG. 17(B)>
In this case, the influence of adjacent outer walls 96 on the rigidity of each outer wall 95 is small. Therefore, each outer wall 95 is arched by a bulging amount smaller than that of the inner walls 88 or is deformed outward of the tubular outer wall portion 93 while maintaining the flat shape. Thus, the distance between each inner wall 88 and the corresponding outer wall 95 is the greatest in the vicinity of each boundary B and is reduced as the distance from the boundary B increases. Accordingly, the distance between each claw portion 89 and the corresponding engaging hole 94 (the distance between the distal end of the claw portion 89 and the outer surface of the outer wall 95) is also the greatest in the vicinity of each boundary B and is reduced as the distance from the boundary B increases. As a result, when the airbag 81 is inflated and deployed, the manner in which the claw portions 89 are engaged with the engaging holes 94 greatly vary widely depending on each claw portion 89.
<When No Slit 97 is Formed in the Tubular Outer Wall Portion 93 (FIG. 18(B)>
In this case, each outer wall 95 is deformed to be arched outward of the tubular outer wall portion 93. Since the rigidity of the outer walls 95 made of synthetic resin is lower than the rigidity of the inner walls 88 formed by metal plates, the degree of deformation of each outer wall 95 is greater than the degree of deformation of the arched inner walls 88. Thus, the distance between each claw portion 89 and the corresponding engaging hole 94 is small in the vicinity of each boundary B and is increased as the distance from the boundary B increases. As a result, in this case also, when the airbag 81 is inflated and deployed, the manner in which the claw portions 89 are engaged with the engaging holes 94 greatly vary widely depending on each claw portion 89.
Therefore, even if the balance of the force by which the airbag cover 85 is engaged with the case 83 is maintained among the claw portions 89 when the airbag 81 is not inflated or deployed (FIGS. 17(A) and 18(A)), the balance is disrupted when the airbag 81 is inflated and deployed (FIGS. 17(B) and 18(B)). This can be one of the causes of lowered inflation and deployment performance of the airbag 81. The phenomenon of imbalanced engaging force can commonly occur both in the case in which slits 97 are formed and in the case no such slit is formed.
To prevent the above described phenomenon from occurring, the rigidity of the inner walls 88 may be raised by increasing the thickness of the inner walls 88 or reinforcing the inner walls 88 with other members, so that they are is less susceptible to deformation. In this case, however, the weight of the front passenger seat airbag apparatus 80 is disadvantageously increased.